This invention relates generally to the field of lasers and more particularly to electrodes for use in CO.sub.2 waveguide lasers.
As is known, CO.sub.2 waveguide lasers have a wide variety of applications. For example, such lasers find use in Doppler velocimeters and navigation systems Thus, it is desirable that CO.sub.2 waveguide lasers have an operating life which is as long as possible. A conventional CO.sub.2 waveguide laser comprises an optical resonator disposed in an envelope filled with a low-pressure, gaseous active gain medium. The gain medium is typically a mixture of carbon dioxide (CO.sub.2), carbon monoxide (CO), helium (He) and xenon (Xe). One or more anode electrodes, typically made from platinum or nickel, are positioned within the envelope Also disposed within the envelope are one or more cathode electrodes, typically made from substantially pure copper. When an electrical potential is applied between the anode and cathode electrodes, an electrical discharge is produced therebetween in the gain medium which induces lasing. The active portion of the gaseous gain medium, that is, the portion of the gain medium in which energy level transitions occur in response to the electrical discharge, is CO.sub.2. The electrical discharge also dissociates a portion of the active CO.sub.2 gas into components thereof, most notably carbon monoxide (CO) and oxygen (0.sub.2). Such dissociation reduces the volume of the active CO.sub.2 gas in the laser, causing a degradation of laser output power and eventual laser failure. Thus, CO.sub.2 dissociation reduces the operating life of the laser.
One solution to the problem of CO.sub.2 dissociation is to periodically introduce a fresh CO.sub.2 -CO-He-Xe gas mixture into the envelope. This is not a practical solution, however, where the application of the laser requires the device to be sealed.
Where the laser is sealed, that is, where the resonator is disposed in a vacuum envelope, the dissociated components (CO and O.sub.2) of the CO.sub.2 must be recombined in order to prolong laser operating life. Carbon monoxide and oxygen do not recombine at room temperature; however, CO and O.sub.2 will recombine under certain conditions when such recombination is aided by certain catalysts. One catalyst which has been used is Hopcalite, a commercially available mixture of magnesium oxide (MnO.sub.2) and cupric oxide (CuO). However, Hopcalite adsorbs a rather large quantity of the gaseous gain medium, and the catalytic activity level of Hopcalite may be difficult to control. Further, free oxygen not recombined with CO by the catalyst oxidizes the copper cathode electrode, thereby forming a relatively thick, and thus fragile, copper oxide layer Portions of such thick coppper oxide layer can flake off and form particles that can disperse throughout the resonant cavity and onto the surfaces of the laser optics, for example, the resonant cavity mirrors. This degrades the output power of the device and limits the useful operating life of the laser.
Another technique which has been used to aid the recombination of CO and O.sub.2 in sealed CO.sub.2 waveguide lasers is to construct the cathode from either substantially pure platinum or an alloy of platinum and rhodium. As is known, platinum is an effective recombination catalyst for CO and O.sub.2. However, a platinum or platinum alloy cathode sputters platinum particulates which are deposited on regions of the envelope adjacent to the cathode during discharge. Such sputtering is a result of bombardment of the cathode by positive ions during the electrical discharge, causing the emission of electrons from the platinum cathode toward the anode. As a result of the deposition of sputtered platinum adjacent to the cathode, the electrical discharge may at times effectively occur from a random one or ones of the platinum deposits, rather than from the surface of the cathode itself. To put it another way, the electrical discharge may randomly jump from the surface of the cathode to a platinum particulate sputter deposit and occur between the platinum deposit and the anode, rather than directly between the cathode and anode. Over time, as additional sputtered platinum particulates are deposited near the cathode, the electrical discharge is given a progressively larger surface over which to occur. Thus, the electrical discharge may effectively migrate randomly about in the area of the cathode from one sputter deposit location to another during electrical discharge. Additionally, the sputtered particulates may become deposited onto the laser optics, damaging the optics and reducing the useful operating life of the laser.